What have you done recently?
if you closed some credit accounts, that can lower your score because it reduces the amount of credit that is available to you, and it will sometimes also shorten your credit history if you closed older accounts. If you made a sudden large purchase on a credit card and have a high balance, that can lower your score until you pay the balance off. If you applied for new credit cards or other new loans, the fact that you applied can temporarily lower your score, although 45 points seems like too much for that. If you received new loans that can temporarily lower your score as well.
remember that your credit score is a crude prediction to the next lender of what kind of a risk you are, and if you are carrying high balances on your existing credit, or if you have recently applied for a new credit, that may suggest that you are a greater risk to the next lender in line.
You can start by getting copies of all three of your full credit reports at www.annualcreditreport.com and make sure all the information is accurate.