There are many ways to produce these emails:
Government mail systems for the White House, the DOJ, and the IRS are documented for review reasons, and have beenfor years. Those emails are held for a minimum of seven years and are directly retrievable from those systems.
The receipients of those emails in departments like Treasury, DOJ, the White House, the FEC are on different servers. The only way that a 'computer crash' affected all those disparate systems, only erasing Lerner's mails but leaving the departments merrily functioning otherwise, is if malware specifically targeting and erasing mails from Lerner was introduced into those systems.
If we assume that by 'computer crash', whatever flack that decided to peddle this lie means a local storage hard drive crash, or a distributed storage array crash, the data is still retrievable - unless the drive was deliberately physically destroyed or repeatedly degaussed. Recovery is not quick, and it's crazy, but it's eminently doable.
So again They are. lying. If Boehner doesn't call a press conference, and announce that NO legislation will be taken up until the material is found and surrendered, than the Republican party is worse than useless. It is an active, complicit, co-conspirator.