You can re-download the book if you have run out of time, so long as the publisher has not completely archived it (which usually occurs on/after the pub date).
If you’re using the NetGalley Shelf app: you can renew your book for another 55 days by tapping and holding the cover image of the book within the app. Then choose Renew. You do not need to press Download on the NetGalley website. Read more here.
If you’re using another supported device or app: Sign back into your NetGalley account, go to your Shelf, click the book title, and then you should see the Send to Kindle and Download buttons.
More information about expiration and archive dates available here.