I assume it is in a pot. Is it in its origional container and if so how long has it been growning in there? It may be root bound and in big trouble. If it has gone dormant for the season get it into your greenhouse keep the soil moist and next season get it out of the pot and into the ground and hope it recovers from its cramped condition. Of course my advice is based upon my own assumption of a potted root bound condition so it would be good to give more details about your plant and it's history.

You don't say where you live, which makes a difference to what you do with it next year. In the UK many people plant the roots outside and train the stems into a greenhouse.
If you are worried the stems have died try scraping a little bark off with a fingernail. It its green underneathits still alive but if its brown that stem has died.

As to cutting back, I would do it between Christmas and New Year, certainly no later than the first week of January. The sap starts to rise early in grape vines, and they bleed badly if pruned when the sap is rising.