Ive been using a ups for about 18 months now, in the last few weeks there have been a few powercuts and my PC has gone off and the UPS has made the alarm sound so not doing its job, it worked fine before although I cant remember when the last time that was. I tested it in all plugs and it seems the backup is not working at all, then yesterday my PC went off while plugged into it bit there wasnt a powercut. Is it dead then and if so how did this happen?

Not an expert in these things but maybe the UPS battery is no longer charging / holding a charge

Almost all issues are battery failure - and at 18 months plus stock time it likely needs a new one. You are aware UPS only provides temporary power to allow turning off computer - it is not designed to run it longer than a few minutes. Larger motorcycle battery supply type stores will have replacement and likely can replace for you if you do not want to open case. If you are comfortable opening be sure to remove power cords first and be careful not to touch wires - just remove the two that attach to battery and remove battery (may have to remove a retaining clip/bar to do so). You can find replacements at larger computer shops and probably HomePro type places.

Or you could just buy a new unit and give the old one away - the major cost is actually the battery on the low priced systems.

Battery

After 18 months, most probably it is the battery.

If replacing, make sure to buy a good quality battery (which could be quite expensive).

If you decide to purchase a new UPS it may be worth looking for one the comes with an RS232 to USB connector

These typically have an app the you can install on your computer which can shut it down normally when the UPS kicks in

In theory this will protect your computer as it gets to shut down normally instead of just powering off when the UPS battery runs out of power

It should also protect your UPS battery as it will only need to provide power to your computer for as long as it takes your computer to shut down

Again others probably have more expertise in these matters than me

99% of all issues with a UPS are solved by replacing the battery. Eighteen months is a bit early for the battery to go but it happens. Replacement batteries available for around 500 baht at any shopping centre with computer or electrical shops. Easy to replace but if you take your UPS with you then shops will often install for free or maybe charge a small fee. Five minute job maximum.

As stated, most likely the battery.

You can get replacement batteries at Amorn stores. I use the shop in Fortune (4th floor). Make sure you know the specs of the battery in your UPS before you head over.