So this problem started apparently by itself and also went away apparently by itself?
The first error, regarding RowGateway, seems to indicate some kind of database problem, but without a fuller backtrace, it’s hard to guess exactly what the cause was.
The second error, involving Composer, is even stranger – not sure why VuFind would suddenly have trouble loading vendor files, unless somebody tried to run a Composer update unexpectedly.
Is there any possibility work was occurring on the server at the time of the error? Do you have any other clues to share?