I have over100 bunches of grapes on my grapevine, which is in my greenhouse. Now a few grapes have turned brown and have dried up. Please can anyone advise me how to save the rest of my grapes .thanks
Have you thinned the bunches? This should be done when the grapes are small.
Details here http://www.readsnursery.co.uk/grapevine-care/
Sounds like grape shanking. One cause is overcropping and with over 100 bunches on one vine that sounds like your main problem. That is an awful lot of fruit for one vine to ripen and I would reduce it by 75%. It can be very hard to bring yourself to remove promising fruit but it really is for the best. Start by removing all affected bunches then remove all small bunches. Reduce any overcrowding where bunches are interfering with each other.
Thinning individual bunches improves the quality of fruit and helps prevent disease so is also worth doing at the right time of year.