Computer turns on but does nothing

Do you hear multiple beep signal from the PC (case)? What's the rest of the setup? Do you have multiple RAMs installed? If you do, then try to use one RAM module at a time and check if the problem persists using only one piece of RAM.

No beeps codes and I have tried the ram one at a time an different ram but still no luck

Also sometimes it does turn on but if you where restart it. It won't boot up if left for a while e.g a few hours

Hp slimline s315.uk
2gb ram
Amd sempron 3600 2Ghz
250 hdd

I had some trouble finding the item you've mentioned. It turns out to be a Slimline 3115.uk, not 315.

Well, how long have you purchase the PC? With 160W of PSU capacity, I'm not really sure it's gonna hold long. My guess is the PSU is failing, since the symptoms you described are pretty much similar to that of a failing PSU.

Ok that's must be it then because I haves tried a lot of things and none of them work thanks

My advice, try using a known good PSU first before you jump to any conclusion. In electronics, nothing is certain unless you've confirm it directly.

Good luck!