This year has been an unusual tax filing season because of the late passing of the tax laws by Congress. The IRS did not have time to rewrite its software and forms. The companies that wrote tax filing software did not have time to test with the IRS on how information on tax returns are sent.
If you filed at the end of January, your problem is probably with the new requirements by the IRS that tax preparers check some boxes on the Earned Income Tax Credit forms.
The error was discovered and fixed within a few days.
For taxpayers that filed before the fix was done, the IRS was sending paper letters asking for information.
You can call the IRS back and ask that the letter be sent to you again. Or, you can ask what information was requested in the letter. Then, you send the information in.
You can also ask for the contact information for the Taxpayer's Advocate, which is a special office at the IRS whose job is to help taxpayers navigate the IRS's bureaucracy.
You will eventually get your refund. If the IRS pays later than it should, you will be paid interest.