Have you tested your water - you know guppies seem to have this reputation of being really hardy but i find them really sensitive think it may be something to do with the excessive inbreeding - i lost 3 that i got the other week was prefect was a bit of a mystery
yeah iv tested the water and its fine, the guppys have been in this tank for a good few months now and there hasnt been anything added (fish, plants etc), water changes every week, they have been eating as normal and never had a problem for months until now just seems strange to me that it came out of no where.