The low limit screw does not back off by itself - it is very unlikely that an adjustment was the problem, as you did not change the rear. Either the larger chainwheel (most likely) or a previous fall that bent the derailleur or hanger inward are more likely. If you found it necessary to adjust the OUTER limit because you could no longer reach the small cog then the latter is likely the problem. In addition to replacing the derailleur both the derailleur hanger and the dropout need to be checked for damage or misalignment.