If they show up without a warrant, you have two choices. You can ask why they want to search your home and then consent, or you can refuse to let them enter. If you don't consent but the police insist on entering your home, stay calm. You don't want to get into a yelling match, and you certainly don't want to get into a physical altercation with the police. You may end up being charged with interfering with a police investigation, obstruction of justice, assault, or some similar offense. It's probably best that you let them in, but make sure you tell them that you're not consenting to the search. And, watch them carefully; keep notes about where they search and what they take.
If the police search your home and a court determines later that the search and seizure was unlawful, any evidence they seized during the illegal search can't be used against you in court. The evidence will be "excluded." This doesn't mean, however, that the state or the prosecution has to drop or dismiss its case against you. The prosecutor may have other evidence against you.
For example, say that a large amount of marijuana was seized from your home. The court later determines that the search and seizure was illegal because the warrant allowed a search of the basement only, but the marijuana was found in the attic. It's likely that the state won't be allowed to use the marijuana as evidence against you. But, the prosecution may have a witness who can testify that she bought marijuana from you several times in the past, or police surveillance recordings and photographs of you selling marijuana. This evidence may be enough to get you convicted.