Have you IPL’d since the abnormal shutdown (with a normal IPL)?
This is pure speculation on my part, but maybe with an abnormal restart there are certain processes that don’t get started (to try to avoid another abnormal shutdown). One of these processes could be the query engine that gets used to find the most efficient access paths for SQL (and SQL RPGLE).
Unless anyone has any better suggestions or explanations I would try and plan a normal IPL as soon as you get a chance.
Sounds like you need to build an index in SQL — I think you can use analyzer on Navigator to help identify this. The system might have built a temp index used it until the IPL wiped it. Indexes look like the keyed part of a LF but process in a much faster query environment.