sql - Suddenly DecryptByAsymKey is returning NULL Values -

hoping out there might have had same experience.

i have asymmetric key use in sql server encrypt field. created key running

create master key encryption password = 'notthep@ssword_just_for_demo' go create asymmetric key mykey algorithm = rsa_1024 go 

for months, working fine, data encrypted, , run

select decryptbyasymkey(asymkey_id('mykey'),tp) tp 

and results (with tp being encrypted field).

suddenly, no reason can discern, giving me null value. there no error, null value.

the data still there, there doesn't seem way decrypt it.

does have suggestions/thoughts on resolve this? copied db, , removed key, removed master key, , recreated hoping perhaps work, no joy.

thanks help.