I recently lost one of my credit cards. I am wondering how it will
appear on my credit report and how it will affect my credit score.
Does this have a negative impact on my credit rating? Also, does it
show as an entirely new account, or does the previous payment history
with that account transfer?

Immediate contact the Idenity Theft clearing House in San Diego. 
Contact all the credit card companies. Put a temp stop on purchases. 
AND pray that an honest person found your credit cards.  OTHERWISE
kiss your life goodbye.

Assuming that no one found your card and charged a bunch of stuff on
it, a lost card just shows up on your record as closing one account
and opening another.  As long as you don't have too many accounts
opening and closing, and effect on your credit should be minimal (I
think).