Hardware conflict?

I have an old computer that runs windows 98 and it has pentium 75 into it.
It used not to have a modem and then i started installing that in. When I slided
in the slot and the computer froze up when I turned that back on. Is that
due to the conflict between the hardware? And now,
the monitor is not even able to boot on for me to test it. Any sugestion, please?
Well, it worked perfectly before I put the modem in.
Thanks