What if my passport expired over 15 years ago, however I am unable to?
For US Passports, yes. However, you will need some secondary proof of citizenship (photocopy of a birth certificate or passport, for example). If your name has changed, you'll also need documents to show that transition. If you have no secondary evidence, you can request a file search for $150, on top of regular fees. This fee is considerably more expensive than ordering a new birth certificate, which typically costs $5-15, but may be useful when you have urgent travel. You can apply at an acceptance facility (like a post office) as usual when going either of these routes, but you should let the acceptance agent know you lack a primary form of citizenship evidence.