Mine looked just like that after I went off on a week's holiday and forgot to stick the cover over the bike
New chain too
Took about an hour with a fine wire brush and lots of WD40 and chain cleaner...back to new again
Only did the outside with the wire brush, used a toothbrush and much patience for the rest as, not being intimate with the inner workings of a chain, aren't there plastic rings in there that are fairly delicate? Either way, whatever I did worked.