First take a deep breath and  be aware that the IRS is not always right ... next review the CP2000 notice carefully against the return  you filed ... are they correct ?  Did you miss any sales or basis  ?   Did they ? 

Respond to the IRS timely either way ...

I GOT A LETTER FROM THE IRS   

https://ttlc.intuit.com/questions/3605673-i-got-a-letter-or-notice-from-the-irs

I AM BEING AUDITED

If you purchased Audit Defense:

https://ttlc.intuit.com/questions/2924451-i-bought-max-or-audit-defense-and-now-i-m-being-audited

AUDIT SUPPORT

https://support.turbotax.intuit.com/irs-notice/audit-support/

FYI ... importing anything is not perfect especially if it was from a broker that puts out a corrected statement after you have filed.  Also some uncovered sales do not always transfer which is why it is important to review everything before filing a return.