Could be one of two things.
Either your hammer isn't cocked, or your bolt carrier group is dirty, and needs to be scrubbed and oiled. Disassemble your rifle and check for both.
If your hammer isn't cocked, cock it. If it won't cock back, something is wrong with the spring and you either need to have it looked at by a gunsmith, or put a new hammer spring in there (if you know how - do not attempt this if you don't know how to do it).
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