This is not a user repairable problem.

You need to take it for a professional repair. The good thing is that this is an all mechanical SLR and was designed so that repairs are usually straightforward. Spares are no longer available, but so many of these cameras were sold and are still in use today that many repairers have a few scrap examples which they can cannibalise when needed.

Given that you can buy a perfectly good used K1000 at online auction sites for between £30-£40 it may not seem like an economical repair, but newly serviced ones with a guarantee sell for over £100 so it's definitely worth it for such a superb classic SLR.