Relative not returning property docs taken 20 years back what to do

20 yrs back, my father borrowed around 1lakh rupees from my uncle, signed a promisory note for the said amount and gave property documents of our home (owned by my mom) as surity. 10 yrs back my father passed away, the amount has not been repaid, nor the documents given back. Now, in 2017, when my mother asked for the property documents to be returned, my uncle is denying the fact that they dont possess any property documents nor have they lent any money to my father. The house is worth 80 lakhs now. How can my mother proceed legally to get the property documents back from my uncle?