The most probable reason is that you have exceeded the
number
of files that
DOS can have open simultaneously. Add
the line
FILES=20
to your DOS
CONFIG.SYS file. This allows DOS to open up to 20
files at the
same time. CONFIG.SYS will only be effective
after you have
rebooted your computer. See the IBM DOS Reference
Manual for
details on the CONFIG.SYS file.