I'm using lubuntu 15 with unity desktop in Acer aspire one netbook.

Suddenly all the minimized windows disappear from the bottom panel so I can't see any icons of the minimized windows on the panel.

When I was trying to find out what went wrong by doing some r/d with the panel, all the right side applets such as volume control, battery meter etc. suddenly packed to left side just after the main menu icon leaving no space at all.
There is an option called spacer that can be added and make space between the applets but all the applets have been set side by side in such a way that the spacer can't be inserted.

I've upgrade & update os(according to various ubuntu fora) but the problem remains.

I've solved the problem except one.
Minimized widows disappear a small widget needs to be installed by right-clicking on a panel. It is "task bar(windows list). All the minimized staff will rest on it.
Solution to my second problem I found when I removed all the right shifted applets one by one and reattached by right-clicking and selecting the applets from the widget list appeared. Widgets attach from the right end of the panel.
I couldn't found the Logout applet that had been with default settings of the panel.