No it is not necessarily an audit.  Random returns are pulled to be processed by hand per IRS procedures. If there was something wrong the IRS would have stated in your letter exactly what they were reviewing.

Don't worry at this point. Your return will be processed and the online status will reappear as soon as they are finished.

There are a few ways you can track your tax return.

• If you filed your tax return through TurboTax, you can view the status of your tax return by going to this site:   https://turbotax.intuit.com/tax-tools/efile-status-lookup/
• You may also track the status of your return by visiting: https://www.irs.gov/refunds
• There is also an app: https://www.irs.gov/newsroom/irs2goapp