The light meter has failed.
You can usually get it repaired (it's even possible to do so on a DIY basis if you're fairly handy, just search "k1000 meter repair", click here
for one immediate possible fix), but it may be cheaper and easier to obtain a handheld light meter instead.
If you do get it repaired then at the same time get a full CLA service (Clean, Lubricate, Adjust) and have the gooey foam light seals replaced. Some repairers may try to charge extra for the latter one-off job, but usually do it for free when you smile and politely point out that you can take your business elsewhere.
Although simple and very basic, a good clean K1000 in a freshly serviced and fully functional condition is one of the only Pentax 35mm SLR's which is still worth reasonable money simply due to high demand for them. If you choose to keep it then it should be good for many more years of flawless service.
I hope that I've helped, please rate my answer.