If your files are in a geodatabase, it may be that it has locked for several reasons. Here's some common ones:
1. It is being accessed by another user. View the geodatabase (carefully) with File Explorer. One of the files is a "lock" file, and part of the filename for that lock file is the computer that created the lock.
2. You are accessing it with more than one process. For example, you have ArcCatalog open separate from ArcMap and both are trying to access the data. Only have one open at a time.
3. The lock is not released from a previous round of geoprocessing or use (even though it should have released). This one is pretty common. I've talked to ESRI Support about this one recently and they admit this is an ongoing issue that they haven't fully resolved. The workaround to get it to release is to run the Compact (Data Management) geoprocessing tool. This tool releases all locks as part of its functioning. Note that this works differently than the "Compact" function in the Administrative tools, which is not as effective.
Chris Donohue, GISP
