The simplest way to get default items back is to reset Theater View. Assuming you do not want to do that (so you do not lose your customization), here is how you get "Search":
Choose the top-level Guide item (the parent of "Guide", "Favorite", "To be recorded", etc.), and choose Add..., Library item. Name it "Search". Under Options, choose "Show search at file level". Do not add any rules. That is it!
To get "Series", add it as above, no Options. Name it "Series". Just under the Name box, click Add, and on the resulting dialog window, select "Library field" on the left, and choose "Series" from the "Select Field" box. Click OK. That is it.
Similarly to "Series", you can also create one for Genre, and other fields too.