Q: Broke golden rule

E-mail in junk folder had a genuine gov.uk Local Authority address, message about contributing to some charities was plausible, opened the attachment, it was blank, realised I'd been suckered, phone the LA who said they were aware of spam trouble that day. No untoward signs on iMac, but haven't done any on-line banking as a precaution. Nto sure what I should be looking out for or whether I should do some sort of cookie clear-out. Any thoughts would be welcome.