As said below, sometimes just thinking about other things for a short time can help writer's block. Sometimes, too, it helps just to get started: keep in mind that what you write doesn't have to be great, just good enough. You don't have to be Shakespeare—though that would be terrific!
You are right in thinking you can't write exactly like Shakespeare: our language has changed significantly in the last four hundred odd years since he penned his last words.
If you are trying to write a couplet in iambic pentameter in a sonnet mode, note that sonnets often express personal emotions, and they can be both serious and comic.
As you know, iambic pentameter uses ten syllables in a line, going from unstressed to stressed. You might write about an event that is happening right now, so I will do that as an example, perhaps in a more lighthearted mode.
The SNOW is FALLing HARD, oh WHY aGAIN?
My FINgers BEND too COLD to HOLD a PEN.
That might not be what you want to end up with, but it is a start.