Take the outer covers off and dig out all the crud with an appropriate tool - use meths or paraffin and an old toothbrush or fine wire brush to remove anything still clinging. Give it a wipe with a clean rag.
Throw the cutting chain away - or clean the existing one and sharpen it - ensure there's oil in the reservoir. Reassemble and adjust the chain tension.
See how it runs...
Peccavi
July 2009