Just got a KC3775 and it has stopped working within a month. what could have gone wrong?

Did you change the battery 3 months ago? Wrong type perhaps?

Was the watch bought new with the battery? Watch in storage and display for 9 months, you had 3 months, time for battery change.

https://www.fixya.com/support/r27181038-changing_watch_batteries

..

is it the battery, batteries are usually always the problem

This is not our product, you are on the wrong website.

It seems to me that the company servicing your timepiece (Rolex or otherwise) must have missed something or did not properly service your timepiece.

You are correct that your watch should not require service every ten months. After servicing, your Rolex, like most all automatics, should run properly for 3-5 years depending on environmental conditions. Our Rolex service offers a one year warranty, and if any issues present within that time, the service is free.

I would recommend using a company that has a full time Certified Master Horologist on staff to service your timepiece that offers a warranty on their work. Thanks for the rating, and we are happy to help you and service your Rolex.

http://www.watchandpen.com/id76.html

Expert watch repair for Rolex Breitling Tag Heuer Movado

try to put a new battery..whether it's only 1 mo old ypu can't tell the watch you bougth has a new battery inside.

It is best to go to a watchmaker for this issue. If the battery was improperly installed, it can damage the movement or contacts, which will mean the movement must be replaced. We see this issue often. Another common problem is that the oils from the installer's fingers may be causing a bad connection if the battery and the contacts are not properly cleaned. We would use the Pulse test to see if the watch has current, if so, the movement is bad mechanically, it must be replaced. If it has no current, we would check and clean all connections and replace the battery. If there is still no current (using Pulse diagnostic) then the movement is bad and it must be replaced.

if the watch is still keeping time but the stopwatch wont work then its probable that the watch has two modules inside,one for the normal hour and min hand and a module to run the stop watch.The cell on the stopwatch module will be flat and needs to be changed.Your watch is waterproof so take care who opens it and ask for it tom be re sealed or it wont be waterproof.
If you cant find a local watchmaker see www.rolexrepairs.co.uk where there is a postal repair service within a day or so.

Best bet is to start by replacing the battery. At 8 months it'll be due for one soon anyway and a new battery may "reboot" it so it runs again. As a battery wears out it doesn't just stop, it may still maintain the display even though it stops keeping time. Also, new "e-ink" displays require no power to maintain the display 0 only to change it.

RETURNED THE WATCH. UNDER WARANTY.

Could be its just not a good quality watch, you didn't state the make. if its an invicta, then tbh, its to be expected they can be very hit and miss. You can buy demagnetizer for about $10, £6-7. Things like speakers, computer monitors or strong electrical field can magnetize a watch making them perform very inconsistently.