Your immediate course of action is to file an FIR for the lost stamp paper in the nearest police station or where you had it last.
It is not about the money but about to prevent misuse of such a stamp paper.
The Supreme Court in Thiruvengada Pillai Vs. Navaneethammal & Anr. (2008) 4 SCC 530, held that a stamp paper, even if it is more than six months old, is valid to be used.
Section 54 just bars taking refund after six months of purchase, but it does not restrict the use of such old stamp paper for an agreement. Thus, nothing prohibits you from using it even after years of its purchase. Law does not have any prescribed period of limitation for its validity.
But Maharashtra and Gujarat are the two states which have specific provisions stating that if a stamp is not used or surrendered back within six months of the date of issuing them, they will be treated as expired.
In most cases seeking a belated refund, reliance is placed on a much-celebrated judgement of the Delhi High Court in Dr. Poornima Advani v. Government of Delhi, WP(C) No. 9014 of 2017, upheld in appeal in Government of NCT of Delhi v. Dr. Poornima Advani,LPA 188 of 2019, wherein the Court permitted a refund of a lost stamp paper on the basis that the State cannot adopt the principle of retention and must not indulge in undue enrichment. However, it is germane to state that Poornima Advani only dealt with whether the case of a lost stamp, not covered by section 49, is a spoilt stamp, and not the case of limitation under section 50.
Reference: Thiruvengada Pillai Vs. Navaneethammal & Anr. (2008) 4 SCC 530;
Section 54 of the Indian Stamps Act,1899
Dr. Poornima Advani v. Government of Delhi, WP(C) No. 9014 of 2017