2007 Volkswagen Polo car won't start

I recently bought a new battery but the car still does not start after driving it. I currently work 10min away from home so the car only drives weekends and not far distances. I have a full alarm system on my car and that also died on me yesterday and the driver side central locking only works the other doors buttons we have to lock manually. It's a second hand car and I have it for 6 months now. When I bought the car it worked fine. When this problem started I had the throttle replaced, bushes on the gears, a new battery and battery cables. I am running out of answers. I had my car at a auto electrician who advised that I should replace the coolant temp sensor but all electrical components of my car is dead. Please help me!

So I'm having a problem with my 1998 VW Polo. The car stopped for the first time and didn't work until we found the problem: the Starter (engine). So we changed it, and it worked well for 2, 3 days until the third day when he (about 2 minutes after started working) started to lose power until dead state. And never turned on again. Can you help me identify the problem or possibly give me some guidance lines so I can solve this problem? Very appreciated and sorry for the bad English but I'm Portuguese and my written English isn't as good as before.

Yes, the starter engage the engine and the spark plugs were testes and they are fine. Battery is all good to. So I'm turning now to the fuel pump or the fuel pump fuse, but I don't know how to proceed with both.