1/ you won't be able to update your APK on the market anymore
2/ you are unable to retrieve details about the certificate used
While there is NOTHING to do for the first problem, there is a way to retrieve the certificate information.
Coming with jdk, you should use jarsigner :
jarsigner.exe -verify -certs -verbose <app>.apk
For each file of the apk, it will give you something similar to
X.509, CN=WillNa, OU=, O=, C=FR
[certificate is valid from 19/12/10 08:38 to 19/12/35 08:38]
this is the certicate details available.