No sound unless a plug is partially inserted into the effects loop jack.

You didn't tell us the make and model of what I'm assuming is a guitar amp? If it doesn't light up and doesn't produce sound it seems like you have blown a fuse. However, fuses blow for a reason and you should take it to a repair shop.

Try inserting a patch cable(or any 1/4" to 1/4" cord) into the effects loop jacks on the rear of the amp. If that helps you have a dirty or bad jack. A little WD-40 works wonders on corroded/dirty jacks.

PRE OUT/PWR IN - (1) Effects Loop - Connect PRE OUT to an effects device input, then connect the effects device output to PWR IN. (2) Multiple Amps - Connect the primary unit's PRE OUT to the auxiliary unit's PWR IN. The primary unit is used to control all auxiliary units. (3) Recording or Reinforcement - Connect PRE OUT to sound equipment's input. Both jacks use a TS (Tip-Sleeve) standard type plug for connections. PRE OUT includes onboard tone shaping.

Assuming the power in is good (I would check that first though, AC voltage going into the amp), then plug a patch cord(any 1/4" to 1/4") cord into the effects loop on the back of the amp and insert and remove a plug into the headphone jack a number of times to clean the contacts(headphone jacks and effects loop jacks are normally switching jacks that pass audio when not in use). Still not working?... If the pots(gain, volume and tone controls, etc.) are scratchy that could be the issue. The input jack could be the cause(damaged, cracked solder or circuit board trace). Wiggle the input jack to see if that causes or corrects the problem. A little WD-40 works wonders on correcting dirty jack issues. If none of the above are the cause then it is likely a cracked solder joint, trace or component lead which would have to be resoldered/repaired (maybe dirty contacts between the small effects board and the main board). Also watch the overdrive channel LED when the audio drops. If the LED goes off could be a power supply issue or a channel switching issue. Tapping on the amp will help indicate bad connections, cracked component leads or damaged traces.

DO NOT try a guitar pedal as an effects insert via the back jacks... they are for an external line level EQ/Effects type unit. Your guitar pedal should NOT have caused any damage. Check the power adapter you are using with the BP200. The BP200 takes a 9 VAC adapter, NOT a 9VDC type common to most pedals. You cannot use one of the Pedal Train type supplies with the BP200. Also check that you have the in's and out's correct on the pedal... backwards and they will cause what you are observing.. Verify your cables are good. The output of the pedal goes in the FRONT jacks of the amp. ONLY line level drives go to the back jacks.

The effect loop is to use exteernal effect module... It is NOT to use guitar pedals !!!! The signals are line levels, NOT guitar type levels. An example of a device might be an Alesis Quadraverb. You take the effect send jack and run it into the inout of the effects module and the output of the effects module into the effects return jack. Many try to use guitar pedals and massive distortion results because levels are too high for the pedals.

No, when you plug into the power amp in that disconnects the feed-through of the preamp.

Usually the loop in is used with an EXTERNAL effects module where the audio signal is "looped" out through the external effect module and then back to the power amp to drive the speaker in the cabinet. The LOOP IN jack would receive signal from the EFFECTS module OUT jack.

The source to go TO the effects module INPUT may come from a jack such as preamp out, or sometimes the same loop jack if it usess the TRS (Tip-Ring-Sleeve) connection method.

If you do not have an external effects module, all this is a moot point in your case.

Well if there is anything you need to insert into the effects loop IE, wah pedals, noise gates etc etc the return is the output of the effect and the send would go to the input of the effect. As far as inputs those are all on the front panel.

The send jack on your amp should be connected to the input jack on your pedal and the return jack on your amp should be connected to the output jack on your pedal. Read the manuals for both the pedal and the amp. If you don't have them you can usually find them on the websites of the manufacturers. Make sure your levels on the amp and the pedal are turned down when you plug them in.