If you look at the long term Nifty PE chart right from 1999 to 2018 you will notice three distinct peaks at Nifty PE level of 27. During the last two peaks in 2000 and 2008 Nifty PE went a bit above 27 and then there was a dramatic fall for a year or two. In Jan 2018 the Nifty PE went a bit above 27 and it looks like it has started to fall down in the last couple of days.
Ofcourse we cannot definitely say that the overall market will fall in the next coming days but the probability of that happening is higher if you look at the previous behaviour when PE was at 27.
If the market does indeed fall dramatically then yes your returns will go down but it will give a much needed opportunity to buy good companies at a fair value. Such downturns are always good news for long term investors. Valuation Screener will help you identify which stocks are undervalued.