Go to File > Restore. Choose the backup you wish to restore and then follow the prompts. You can enter all of March and April and then run bills using a date selection with end date of March 31st. This will pick up just the March time. Make sure you set the bill date to be the date you want for the invoices. Then approve the bills and run the April bills. Please note that if you are recreating invoices run previously they may not have the same invoice number they had previously. 

How do I use the date selection with the end date of March 31?

I've reentered all of my information for March.  How do I set it up to just print from March 1 to March 31?

In generate bills choose transaction date from left column and then set the date range.