Longines Customer Service link

http://www.longines.com/contact/faq/customer-service

They have some FAQs they have posted the answers to on the web page.

REPAIRS
1) Where should I send a Longines watch that needs to be repaired, adjusted or serviced?
If your Longines watch needs to be repaired, adjusted or serviced, we recommend that you bring or send it (well padded and by registered delivery in the latter case) to your nearest Longines authorized service centre. You will find its address in your guarantee booklet or on our website in the Customer Service section.

2) Is the repair of a Longines watch that is more than 20 years old still possible?
Yes, we still repair vintage watches at our head workshop in St-Imier.

However, without having the watch to hand, it is impossible for us to judge its state and to know if all the necessary spare parts for the repair are still available.

Therefore, we kindly suggest that you send your watch via the nearest Longines authorized service centre to the Longines head office in Switzerland. Upon receipt and after analysis of your watch, we will send you our comments and a price estimate for the repair of your watch.

The website itself

http://www.longines.com/watches/collections