Thank you for your question.
The only way you can recover your lost tax id, whether it is business or personal, is to contact the IRS at(NNN) NNN-NNNN and to then select option one and option four. when you get a live agent, ask them to speak to someone about retreiveing your lost tax id number.
After verifying your identification, they will send verification to the address on the account. They will not give it over the phone.
An alternative way, is to find any tax communication you may have recieved from the IRS. Your tax ID is part of the address code that appears over your name or the name of your business on the envelope or the top of the communication.
Also your tax ID is part of the documentation for all of the following:
drivers license (it may be on your driver's license)
leases for cars and apartments
if a student it would be on student documents
If you are an F-1 student it will be on your form I-20
It may appear in some of your immigration paperwork if you are a non-resident.