Oui, je me demandais s'il t'en restait

‘Il reste …’ is an impersonal and invariably 3rd person singular verbal  expression ‘it/there is … left’ (it/there ‘remains’…..)  

https://www.lawlessfrench.com/grammar/impersonal-verbs/

 “Il n’en reste plus” .. there is none (of it) left. The ‘en’ for of it/them is essential in French and must be included. 

I was wondering if you had any (of it) left/remaining   - Je me demandais s’il t’en restait. Past tense, imparfait in French.

 (Direct literal translation more like ‘I was asking myself if there was any of it left/remaining to you’ which sounds quite unnatural  in English). 

The following short YouTube clip might help. It also covers the variant ‘il me reste …’ , which is also a form of the expression that is commonly used.

https://www.youtube.com/watch?v=NLw9Hc2xqsg