Is there any way to recover the key??
Theoretically, yes... realistically, No.
Given the keyspace is finite, then by trying every combination you would eventually find it. However, given the keyspace is so large, the sun is likely to have died and all life on Earth extinguished before you can test them all.
How did you originally generate this "watching only" address? Did you originally have the private key held in a different wallet.dat file that has now been lost?