Emails disappeared - unable to access on iPad or Rogers account - only via MacBook

Hello!

All of my emails disappeared up until July 2015. I was unable to access these emails via my iPad (Mail app) and through my Rogers account online.

However, for some reason I can still access those emails on my MacBook air using the Mail app. The Mail app is saying that it is offline, and I am afraid that I will lose all of those emails if I switch it to online. Is there any way to restore these emails so that I am able to access them on my iPad or from any other device? Any suggestions would be great!