I am sorry to learn of this situation.
Unfortunately, the notice provision in the PA Storage Facility Lien statute does not require that you actually receive the letter - it only requires that notice be mailed via certified mail to the owner of the locker at the last known address.
(You can find the statute in full here: http://www.bbb.org/pittsburgh/migration/temporary/2013/09/publications-for-consumers/pennsylvania-storage-law/)
However, if you find that even after reviewing the statute and the date of mailing of your letter that the storage facility did not provide you with sufficient notice, you can consider suing them for breach of contract, violation of the above statute, and negligence.
You say that your cost of replacement is approximately $50,000.00 so you will be in general civil court (this is a little more complex, most parties are represented by attorneys, and it takes longer - often more than 12 months), but you can prosecute these claims on your own - plan on visiting your law library for help with sample pleading forms and information.
Short of filing a lawsuit, you can try to mediate the dispute with them - contact your local bar association and request referrals to mediators, a third party neutral can often help you reach a mutually agreeable resolution. Use the bar association's referrals to contact a mediator or two, the mediator will then contact the other party to set up a mediation session, and you can go from there - hopefully resulting in a formal or written settlement agreement, and save yourself the time and expense of litigation.