There could be a number of reasons for your problem playing this game.
The most probably is that you have a normal (non-stealth - 4/5 wire) chip in your psx and the game is using some kind of antimod trick where it looks to see if you've got a modchip in yer system. If these games find one they will either let you know by a message on the screen or they'll just freeeeeeeze!
There are two ways around this problem: Either fit a stealth (7 wire) modchip to your psx or get a patch (crack) for the game from somewhere like gameCopyWorld. They'll give you full instructions on how to use the patch too.
If you already have a stealth chip in your psx then it is probably as Mattel007 said: Your laser unit is shafted!!! This will cost you big time to fix so you're probably better of buying a second hand psx!
Hope this helps!