-How many Invoices do you think are missing?
-What do the missing Invoices have in common? Are they sequentially numbered or the same date?
-Does it look like the missing Invoices could represent a batch?
-If so, has an A/R batch been deleted? Print the Batch Status in A/R and look for a Deleted (or Open) batch.
-Did you experience any errors during Day End processing at any point?
-Have you restored any data files from backup recently? If so, did you restore all files or only A/R? A/R, I/C and O/E should be restored together.