What does this error mean? It's preventing me from looking at my Accounts/Leads on my main page.
This sounds like a possible missing custom field in one or both of those modules. Something may be trying to pull data from it, another field with a formula (or dependency) that used that missing field perhaps. If the field was deleted in Studio, it will no longer be visible there, but if you look at the fields with calculated values, you may be able to identify the cause.
Slightly more advanced, it's possible that the column still exists in the database. This could help identify what kind of values where used and then help identify which field(s) were using it. For example if the values are a currency, then it's likely another field (or multiple fields) that's a currency as well.
Retrieving data ...