The screws of the SL12xx are notorious for breaking. I've had it multiple times (I repair those things) over the past few years. Once the head breaks off you can remove the bottom part. If you're lucky there's enough thread sticking out to grab it with a plier and unscrew the remaining part.
When the head is worn down to much so a screwdriver doesn't grab it anymore, use pliers to grab the screw head and try to get it loose.
New screws can be purchased or grabbed from a universal screw set/kit. You can check this topic : Opening a Technics SL12xx
when you want to know which screws goes where.