You can restore damaged drawings by copying backup files into new drawings.
When you save a named drawing for the first time, AutoCAD LT creates a backup (.bak file extension) copy of the drawing. Each time you use SAVE or SAVEAS with the current drawing name, AutoCAD LT updates the backup file. You can turn off this automatic backup feature with PREFERENCES. If you want AutoCAD LT to automatically save your drawing to a file, you can use PREFERENCES to specify a time interval between save operations and a file name for the temporary drawing.
If your program terminates unexpectedly, AutoCAD LT tries to rename the backup file to prevent the corrupted file from replacing the existing backup file. AutoCAD LT renames the BAK file with the file extension .bk1 if that file name does not exist. If that backup file does exist, AutoCAD LT generates new file extensions in the sequence .bk2 through .bk9 and .bka through .bkz until it finds a unique file name.
If your system fails and the BAK file contains the previous version of the drawing file, copy it to a safe place in case you need to revert to that version. To use the backup copy of the drawing, copy it into a DWG file.