Presuming you mean the outward portion of an Off-Peak Return from Nuneaton to Grantham, the answer is "it's complicated".
In principle it's a valid route (you can reach Nottingham via Tamworth or Leicester), and there are no break of journey restrictions on this ticket - so there is certainly no problem with exiting at Nottingham on the first day of validity of the outward portion.
However, recommencing travel on the second day of validity, when using the outward portion of an Off-Peak Return, is only permitted "if the journey cannot be completed on the same [first] day". This has long been an ambiguous definition - does the word "cannot" refer to a journey that can't be completed within the same day because you set off too late to make the last connection all the way to your destination? Or does it only refer to journeys that take so long it's physically impossible to complete them within a day? It's unclear.
Whilst some staff will be familiar with the provisions for overnight break of journey on the outward portion of an Off-Peak Return, many will not and will claim that the ticket has already expired. Therefore, I don't think it's something I would advise for a journey as short as this, unless you are willing to accept the potential risk of being charged for a new ticket (or potentially issued with a Penalty Fare or worse).