Is it a homeowners twig chipper/leaf grinder? I'm thinking of the Craftsman or MTD style idy-biddy yard maintainer style.
If it is that, pull the knives out, and get them professionally sharpened - chances are they never have been. If you then want to do it yourself, take very careful note of the sharpening angle, the clearances to the anvil and keep your fingers clear. Maybe pull the plug wire off while working on it - you never know what the previous owner had rigged onto it.
Check the websites for the above brands - I suspect that there were only several manufacturers of these things, and they just got painted and branded differently. Find one that looks like yours, and order their owners manual.