Unless its a bigger commercial type one its probably not worth it and parts can be very difficult to get hold of. There's various people in Liverpool sell brand new Karcher K2's for ¬£35 to ¬£45. https://www.facebook.com/marketplace/item/2336418833139544/
and Karcher HK 7.5 on ebay for around ¬£40 as well.
It could be a silly simple fault but usually you will still need a part.
Tried to repair mine not long ago, the metal holder's for the carbon brushes had rusted and jammed, after a lot of faffing around I got it going but it hasn't lasted, there was too much damage to the carbon brush and the holder. Previous washer (different make) had the same problem and burned the armature out.