You have tried to exceed the maximum number of indexed files which you can have open at any one time. This can be a software or an operating system restraint, but you must not violate it.
Resolution:
Close some of the open indexed files which you are not currently accessing, and then try to open the relevant file again. You should then be able to continue to run your program. (Indexed files count as two files, one for data and one for the index.)
