I have a APC Back UPS, model UPS ES 500.

Today when I was turning on my PC, suddenly after flashing yellow light ( which is normal ) it started fashing red-light ( instead of green light ) and was beeping violently. After 3 to 4 trials it finally came back to normal. My question is, what was this problem ? Why did it happen ? Also, recently we are have sudden voltage fluctuations in our Apartment, could this fluctuations trigger this problem ?

Thank you.

The problem encounters when the charging is about to finish in the battery of UPS. But, as you said, voltage fluctuation did occur, might cause the battery failure.

It happened now as well, but there wasn't any voltage fluctuations. What should I do ?

If it's under the warranty then contact APC and see if there might be a software issue.

Another issue could be (depending how old it is) your battery is no longer able to hold a charge. UPSes can be very finickey as they get old, so really you might need to buy a new unit depending on its age.

Its not under warranty.

It now flashes yellow and green light with two beeps and after 10 mins it stabilizes to green.

The problem is either with the device itself or the battery. I have worked with many of these smaller units and batteries do not last for many, many years. It would be good if you had access to another battery so that you can swap it out and see if that resolves your issue.

Alternatively, if you have a local computer store, they may have a tool that can be used to test the device.