McIntosh amplifyer problem

If it is a screw that threads into one of those terminal strips for
connecting speakers?
They do on occasion get worn and the screw doesn't thread in properly any
more.
It may be possible to carefully rethread it for slightly bigger screw! Or
even try (very carefully) a slightly bigger screw and make it cut its own
new thread. But be careful; too much force, danger of cracking the terminal
strip insulator!
If not take the amp. apart. Install a new terminal strip and
reconnect/resolder etc. the wires to it 'exactly' as it is now.
Make sure none of the wires is touching anything else or the metal frame of
the amp.
If the terminal strip is riveted to the chassis it will be necessary, to
very carefully (and watch out for metal drillings falling into the amp.)  to
drill out the rivets and probably use screws or rivets, to reinstall a new
terminal strip.
If you don't have the tools and skills to do this get a competent technical
person to do it.
Not having seen the situation; possibly estimate a couple of hours; after
finding the right parts, to repair and test it (provided it is working
before hand!).
But as with all things technical/electronic you never know what you may find
inside of the unit!