It's been a while, but if I can remember... Do you get a flashing C:\ or A:\ prompt showing on the screen? If so, type dosshell, and that should take you into the operating system shell. Most likely you have a bad command in the autoexec bat file. I'm going to stop here, because if you havent gotten this far... all of the above is redundant
If the first part doesn't work, then when the system starts to boot up, press the F8 key (?I think or F2) and that will take you into the bios setup mode. There you will either be asked a series of questions about time and date, or you will be given a set of options you can adjust like memory size, hard drive size, monitor type, etc. It may also tell you there is an error somewhere and ask you if you would like to correct it. Answer yes and save changes and reboot. That should fix it.
If not insert the dos startup disk into the floppy and reboot. Hopefully you should get a "A:\" prompt on the screen.
Type C:\ and hit enter. C:\ should now be flashing. Type "CD:\dos" hit enter. Then type "dosshell" If DOS was properly installed then a screen should come up, if not, you probabily need to reload DOS. (do not include " "quotation marks in the typing)
Sorry for the rant, and I hope it makes sense... but I'm trying to recall as I go, and it's been a long time since i messed with DOS.
HTH
Paul