Hello, nothing happens when I turn on my 9780. If I plug it in a red light comes on, if I plug it in without a battery and wait a few minutes the red light goes off and a battery error symbol comes on. When I put the battery in after getting the error symbol, the screen goes off again and the red light comes on for a minute or two. The red light then goes off, I left it to charge for 1-2 hours in this state, on my return it was the same as before, still not working. I don't have any friends with a similar battery, meaning to try a different one would mean buying one, which would be one of my last resorts.
If anyone could make some suggestions then it would be of great help, thanks in advance.

Have you let the battery run down past 20% more than a few times? If so, you may have damaged the battery.

You can try removing the battery, placing the device on a wall charger and then insert the battery and see if it will charge. Typically, you need to get past a certain point, around 10-15% charge before the device will turn on. That might take an hour or two.

If that fails, try a new OEM battery made for your specific model.

Yes well, I will buy a new OEM battery tomorrow and try leaving it on charge with the battery in tonight. Since the screen is functioning (to display a battery error), what are the chances that the phone has hardware damage (not including battery)?
Thanks!

Depends. Did you drop it or get it wet?

Neither, I plugged it into my laptop when the laptop was crashed and from then on it stopped working. It may not have been this that caused it, as it could be coincidental that the battery failed at a similar time to me plugging it in.

A new battery should correct the problem. Avoid draining it past 20%

I tested the battery, it has enough voltage across it to power a small lightbulb but somewhat inconsistently.

Then you should heed this advice.

Tried 2 different batteries, still no luck.

Ok , if you tried new batterys and if they were OEM Blackberry batterys then it may be the phone. check the connectors on the phone and make sure they make contact and are clean. Failing that it may require repair.