hebe has colapsed in middle help

When you say 'collapsed' do you mean died or flopped? If bits have died then you should cut them out. If it has flopped then you could support it with twigs or wait for new shoots to regrow into the space. Hebes do eventually get very leggy as they get older and are not really able to be put back to their original shape. Try taking cuttings and then replace the old plant with a new one in a year or two's time. Or buy something new and exciting to replace it!