More information is needed since more than one ransomware uses the .arena extension.
uses filenames with hexadecimal strings (i.e. 0D0A516824060636C21EC8BC280FEA12.ARENA
.arena Dharma (CrySiS)
uses filenames with id-<8 random hexadecimal characters>.[email] (i.e. .id-BCBEF350.[firstname.lastname@example.org].arena
Neither are decryptable without paying the ransom. If possible, your best option is to restore from backups or wait for a possible solution at a later time.
Did you submit any samples of encrypted files and ransom notes to ID Ransomware
for assistance with identification