Cannot use the search function any longer

I have bought 2 ebooks a while ago. When an ebook was loaded, I could see a page and some icons top right where I could use the search function, among others. These icons have vanished now, and searching is no longer possible. This renders the ebooks almost worthless because they are dictionaries with more than 1800 pages each. The volume itself exceeds 100 MB. Is there anybody who could help me out?