My ps2 doesn't open enless I hit it on the side can I fix it

It can be fixed that's for sure, have you checked inside if there is a lot of dust or something that shouldn't be there (a small piece of plastic etc)?

Also if it the original PS2 or the slim one, they open differently?

The big balky original on I have cleaned it and I found a lose of plastic piece in it

So do you still have the problem since you found the loose piece?

If so I'm guessing something is out of alignment and when you hit it the disc is able to come out once again. It you have it sitting horizontal stand it vertical and see if that fixes it (or vice versa if you have it vertical now).

Option 2 is to take it to a best buy or something like that and they can probably replace the disc tray

if there's no disc in it I have to hit it but when there's a disc its opens the only resone I care is I want to sell it and get a x box 360 will this cost me