you've actually said the problem.

the belts slipped.

You need to tighten the grubscrew on the stepper motor gear.

They don't slip, but the drive gears/cogs/pulley things can often work loose.

And for the record it IS an original prusa i3 design.
With a bit of work, it's a really nice little machine.

If it's got the plywood frame and the crappy control panel with buttons - I've got it's twin sitting on top of my filament cupboard.
Awaitign the day i get round to fitting a 32bit board and touchscreen to it.

I do recommend fixing it to a solid flat surface. Mines bolted to a sheet of 5mm aluminium. But anything solid and flat will work.
reason being the xy gantry really isn't attahced to the print bed very well. So that can recult in uneven bed travel.
So bolt it to something solid and flat.

Other than that and the horendous screen and controller board - the firmware of which you cannot update or change in any way. - hence the new screen and board.
It's good kit.
Mechanically and design wise it's a good machine. Just let down by the electronics.

I did do some porints with it - primarily to see what you could do with s sub £100 machine.

Reminds me - is it one of these ?
https://3dprintboard.com/showthread....100-3d-printer

if not ignore what I just said :-)