I replaced my crankshalf sensor and now it wont start on 01 buick lesabre 3800s 2 engine?

Put the old one back on,maybe you just got a defective sensor.
If it runs after changing back make sure you do replace it with a new good one as they fail often.

Did you do a
Crankshaft Position System Variation Learn ?
The crankshaft position system (CKP) variation compensating values are stored in the powertrain control module (PCM) non-volatile memory after a learn procedure has been performed. If the actual crankshaft position system variation is not within the crankshaft position system variation compensating values stored in the PCM, DTC P0300 may set, refer to Diagnostic Aids for DTC P0300 .
The Crankshaft Position System Variation Learn Procedure should be performed if any of the following conditions are true:
• DTC P1336 is set
• The PCM has been replaced
• The engine has been replaced
• The crankshaft has been replaced
• The crankshaft harmonic balancer has been replaced
• The crankshaft position sensor has been replaced